Suspensions zeta potential

Response to Electric and Acoustic Fields. If the stabilization of a suspension is primarily due to electrostatic repulsion, measurement of the zeta potential, can detect whether there is adequate electrostatic repulsion to overcome polarizabiUty attraction. A common guideline is that the dispersion should be stable if > 30 mV. In electrophoresis the appHed electric field is held constant and particle velocity is monitored using a microscope and video camera. In the electrosonic ampHtude technique the electric field is pulsed, and the sudden motion of the charged particles relative to their counterion atmospheres generates an acoustic pulse which can be related to the charge on the particles and the concentration of ions in solution (18). 

Streaming Current Detectors These units produce a measurement closely related to the zeta potential of a suspension and are used successfully in optimizing the coagulant dose in clarification applications. 

Flocculating agents can be simple electrolytes that are capable of reducing the zeta potential of suspended charged particles. Examples include small concentrations (0.01-1%) of monovalent ions (e.g., sodium chloride, potassium chloride) and di- or trivalent ions (e.g., calcium salts, alums, sulfates, citrates or phosphates) [80-83], These salts are often used jointly in the formulations as pH buffers and flocculating agents. Controlled flocculation of suspensions can also be achieved by the addition of polymeric colloids or alteration of the pH of the preparation. 

Suspensions are generally evaluated with respect to their particle size, electrokinetic properties (zeta potential), and rheological characteristics. A detailed discussion on the methods/techniques and relevant instrumentation is given in Sec. VII. A number of evaluating methods done specifically with suspension dosage forms, such as sedimentation volume, redispersibility, and specific gravity measurements, will be treated in this section. 

The determination of the zeta potential of particles in a disperse system provides useful information concerning the sign and magnitude of the charge and its effect on the stability of the system (see Sec. II.B) [56, 206 208], It can be of value in the development of pharmaceutical suspensions, particularly if the... 

JB Kayes. Pharmaceutical suspensions relation between zeta potential, sedimentation volume and suspension stability. J Pharm Pharmacol 29 199-204, 1977. 

One of the most difficult parenteral dosage forms to formulate is a suspension. It requires a delicate balance of variables to formulate a product that is easily resuspended and can be ejected through an 18-to 21-gauge needle through its shelf life. To achieve these properties it is necessary to select and carefully maintain particle size distribution, zeta potential, and rheological properties, as well as the manufacturing steps that control wettability and surface tension. The requirements for, limitations in, and differences between the design of injectable suspensions and other suspensions have been previously summarized [17b, 18,19]. 

Initial studies were made with the Rank Bros, electrophoresis unit, using the dilute supernatant suspension over a dispersion of 3.33g of carbon black per liter of dodecane equilibrated for 24 hours with the added 0L0A-1200. The electrophoretic mobility (u) of 1-3 pm clumps of particles was observed at a field of 100 volts per centimeter. The zeta-potentials ( ) were calculated... 

In another application, the magnitude of the zeta potential is measured as a function of added counterions. The variation in zeta potential is found to be related to the stability of the colloidal suspension. The results of a gold colloidal suspension (gold solute) are reported as follows ... 

In various kinds of industrial production, materials need to be treated with charged colloidal particles. In such systems, the value of the zeta-potential analyses are needed to control production. For example, in paper, adhesive, and synthetic plastics, colloidal clay can be used as filler. In oil drilling, clay colloidal suspensions are used. The zeta potential is controlled so as to avoid clogging the pumping process in the oil well. It has been found that, for instance, the viscosity of a clay suspension shows a minimum when the zeta potential is changed (with the help of pH from 1 to 7) from 15 to 35 mV. Similar observations have been reported in coal slurry viscosity. The viscosity was controlled by the zeta potential. 

Figure 3 Plot of zeta potential vs. pH for a typical mineral suspension...

Recent advances made in measuring particle charge and mobility in nonaqueous suspensions are reviewed. Microelectrophoretic techniques have been used to determine zeta potential and the measurements related to particle stability. 

A foaming agent, such as crude cresol or pine oil (soap is unsuitable, as it lowers 0 too much), is added to the suspension of ground ore and collector oil in water and the pH is adjusted to give the particles low zeta potentials and, therefore, minimise electrostatic repulsions. Air is forced through a fine sieve at the bottom of the vessel. The particles of metal ore become attached to the air bubbles, which carry them to the surface (Figure 6.7), where they collect as a metal-rich foam which can be skimmed off. 

The first four methods are described in Refs. [81,253,254] and the electroacoustical methods in [130,255-257]. Of these, electrophoresis finds the most use in industrial practice. The electroacoustic methods are perhaps the best suited to studying concentrated suspensions and emulsions without dilution [258], In all of the electro-kinetic measurements, either liquid is made to move across a solid surface or vice versa. Thus the results can only be interpreted in terms of charge density (a) or potential (zeta potential, ) at the plane of shear. The location of the shear plane is generally not exactly known and is usually taken to be approximately equal to the potential at the Stern plane, = W d), see Figure 4.9. Several methods can be used to calculate zeta potentials [16,81,253], Some of these will be discussed here, in the context of electrophoresis results. 

Size enlargement of fine particles in liquid suspension can be accomplished in a number of ways. Electrolytes can be added to a suspension to cause a reduction in zeta potential and allow colliding particles to cohere. Examples include the use of trivalent aluminum and iron ions to flocculate the particles responsible for the turbidity of many water supplies and the flocculation of metallurgical slimes by pH adjustment to the isoelectric point. Alternatively, polymeric flocculants can be added to suspensions to bridge between the particles. A wide range of such polymeric agents [1] is available today to aid the removal of fine particles from water. 

V. M. Gun ko, V. I. Zarko, R. Leboda, and E. Chibowski, Aqueous Suspensions of Fumed Oxides Particle Size Distribution and Zeta Potential, Adv. Colloid Interface Sci. 91(1), 1-112(2001). 

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